Acinetobacter baumannii is a nosocomial pathogen responsible for an increasing number of infections in VA, civilian and military hospitals. This proposal will address a novel mechanism that regulates virulence in A. baumannii. This mechanism involves a molecular switch that interconverts A. baumannii between virulent and avirulent states. Cells in the virulent state grow as opaque colonies when viewed by indirect oblique lighting. In contrast, the avirulent form produces translucent colonies that are rapidly killed in-vivo by host defenses. The central component of this regulation is a transcriptional regulator (ABUW_1645) that acts as a bistable switch to drive the differentiation from opaque to translucent states. In addition, a peptide-based quorum sensing system activates the expression of this regulator through a two-component system when cells approach high density. The completion of our work will result in a greater understanding of this novel regulatory pathway that controls virulence. This information along with the strains constructed in this proposal will pave the way for the development of high- throughput screens to identify small molecules that either drive cells from the opaque (virulent) to the translucent (avirulent) state, or block the ability of translucent cells to switch to opaque. Ultimately, these inhibitors may result in the development of novel therapies to disable virulence in this increasingly important bacterial pathogen.